1. Field of the Invention
The present invention relates to an optical linear encoder used for the measurement of positions in machine tools or the like, and in particular to a position detector using moire fringes produced by diffraction gratings.
2. Description of the Prior Art
Since moire fringes obtained by overlapping two diffraction gratings are sensitive to relative changes in a lateral direction and a counting measurement of displacements in fine steps can be made, they have been widely used in methods of measuring length.
Two diffraction gratings (hereinafter referred to as "a first grating" and "a second grating") are mounted on two portions which are displaced relatively to a machine and therefore they must be suitably spaced apart from each other at all times. Meanwhile, when the grating pitch of each of the above-mentioned diffraction gratings is made smaller in order to increase the resolution for length measurement, the influence of the diffraction effect of the light becomes larger. As a result, the shadow of the first grating on the second grating becomes thinner because of the diffraction effect and direct moire fringes with high visibility cannot be obtained. Hence, diffraction moire utilizing a Fourier image has come to be used. That is, when the first grating is irradiated with parallel luminous fluxes whose phases are uniform, the bright and dark distribution (light distribution in which brightness and darkness are reversed is made at a position of a multiple of half-integers) of light having the same pitch as that of the grating spaced an integral multiple of the distance at which two times the square of the pitch P of the grating is divided by a wavelength .lambda. is made behind the first grating 1 because of the diffraction effect of the light, and this reproduced bright and dark distribution of light is called a Fourier image. If the second grating is placed at a position where this Fourier image is formed, the diffracted light from the second grating will have a sharp contrast of a cycle P with respect to the relative displacement of the two gratings, which is called diffraction moire in the lateral direction. Applications of a relatively short measurement length as in mask alignment in micromachining such as the fabrication of semiconductors, etc. have been investigated (For instance, p. 984, J. VAC. SCI. TECHNOL. 15(1978), and p. 1276, J. VAC. SCI. TECHNOL. 15(1983)).
On the othr hand, when the length to be measured is made longer, the grating pitch P is made smaller, and the length measuring accuracy is made higher, a length 2P.sup.2 /.lambda. in which a Fourier image can be made becomes abruptly shorter in proportion to the square of the grating pitch P, with the result that it becomes difficult to hold the two diffraction gratings in the gap in which a Fourier image can be made, with high accuracy over a longer distance. When the gap of the grating deviates from the position at which a Fourier image can be made, the intensity of the diffracted light varies greatly and the determination of position becomes impossible. For example, if it is supposed that the grating pitch P is set to 1 .mu.m and a wavelength .lambda. of 0.633 .mu.m is used, then the gap G of the grating must be kept at sufficiently small variation with respect to the 1.6 .mu.m that provides a Fresnel number ( .lambda..multidot.G)/P.sup.2 =2 which is obtained by dividing the product of the gap G of the diffraction grating and the wavelength .lambda. of light by the square of the pitch P of the diffrction grating. For this reason, diffraction moire cannot be used in a length measuring method with high accuracy in ordinary machining tools or the like.
Regarding such circumstances, a position detector capable of detecting position with high accuracy by obtaining diffraction moire signals sensitive to a displacement in the lateral direction without being influenced by changes in the gap of the first and second gratings has been disclosed by this applicant (Japanese Patent Laid-open No. 17016/1986).
This position detector is designed to obtain a signal equivalent to the average value of diffraction moire signals by varying the optical path length of the gap between the gratings and detect the position using a signal change, in which half the pitch P of a diffraction grating which appears at this average value, is used as a cycle, in each of the portions of the effective area where the first and second gratings face each other.
FIGS. 1 to 3 are perspective views each illustrating one example of the above-mentioned averaged diffraction moire position detector. A case where zero-order diffracted light is used will be explained below.
In FIG. 1, first, a first grating 1 is irradiated with a laser beam LB and a transparent plate 3 having a staircase-like step-difference is mounted on a second grating 2 placed behind the first grating 1. The transparent plate 3 is formed from a material having a high index of refraction which is stepped so that the range of the gap G becomes G.sub.o to (G.sub.0 +2P.sup.2 /.lambda.) optically. An optical path difference is given to each portion of the laser beam LB by means of the transparent plate 3 having the step difference. Since the range of the optical distance 2P.sup.2 /.lambda. is divided into five by the transparent plate 3 having a step difference in FIG. 1, the transparent plate 3 is constructed in a staircase form of five steps. A lens group 4 arrayed one-dimensionally behind the second grating 2 collects luminous fluxes that have passed through areas which have been divided into five areas different in optical distance. Light which has been collected by the lens group 4 is detected by a photodiode group 5 independently of others. Thereafter, a signal from the photodiode group 5 is added by an adder 7 composed of an operational amplifier, etc. and a displacement signal is obtained.
In FIG. 2, the first grating 1 and the second grating 2 are placed in parallel to each other and a random optical-path difference plate 9 is mounted on the second grating 2. This random optical-path difference plate 9 is formed from a transparent plate which has been irregular at random in one side and has been plane in another side so that the optical-path difference in each portion of the laser beam LB becomes random in the range of 2P.sup.2 /.lambda.. Each of the laser beams LB is collected on a diffusion plate 10 independently of the others by the lens group 4, and it is arranged that the focal points of the lens group 4 are arrayed in one row on the diffusion plate 10 without overlapping. Each of the luminous fluxes where the laser beam LB is collected becomes coherent light by the diffusion plate 10. Light diffused by the diffusion plate 10 passes through a convex lens 11 and is detected by an optical sensor 12 such as a photodiode. Because the diffusion plate 10 is used, luminous fluxes which have passed through different gap optical-path length are averaged without interfering with each other.
In FIG. 3, the first grating 1 is positioned perpendicularly to the laser beam LB, and the second grating 2 is positioned inclined with respect to the first gratings 1. The gap between each of the first gratings 1 and each of the second gratings 2 is so adjusted that it includes the range of 2P.sup.2 /.lambda. in an area where each of the first grating 1 and each of the second gratings 2 face each other. Of the light beams which have been transmitted through each of the first gratings 1 and each of the second gratings 2, only the zero-order diffracted light L.sub.0 enters the light-receiving surface of a photo detector 13 positioned behind and is detected.
In each of the above-mentioned averaged diffraction moire position detectors, a position is detected accurately in such a way that the amount of light obtained at a certain position, i.e., an electrical signal converted by a photo detector or the like, is detected and corresponding position data is output, or corresponding position data is output based on the relationship among a plurality of light amounts whose phases are different from each other.
However, when, for example, the amount of light generated from a light source changes due to some reason, the light amount will not be fixed at the same position and an error will occur regarding the detection of a position.
FIGS. 4 and 5 are perspective view showing an example of the optical system of the averaged diffraction moire position detector which eliminates the above-mentioned drawbacks and a block diagram showing an example of the detector's processing circuit, respectively. The first gratings 1 are positioned perpendicularly to the laser beam LB, and the second grating 2s are positioned inclined with respect to the first gratings 1. The gap between the first gratings 1 and the grating sections 2A and 2B of the second gratings 2 is so adjusted that it includes the range of 2P.sup.2 /.lambda. in an area where the first gratings 1 and the grating sections 2A and 2B of the second gratings 2 face each other. Of the light beams which have been transmitted through the first gratings 1 and the grating sections 2A and 2B of the second gratings 2, the zero-order diffracted light is collected by a cylindrical lens 23, enters the light-receiving surfaces of measuring photo detectors 14A and 14B positioned behind and is detected. The phase of each grating line of the grating sections 2A and 2B of the second gratings 2 deviates in the direction of displacement, and electrical signals of different phases are generated in the measuring photo detectors 14A and 14B to which the light beam which has been transmitted through each of the grating sections 2A and 2B as the second gratings 2 is displaced. Meanwhile, a transmission section 17, which transmits a narrow light beam in the longitudinal direction of the first gratings 1, for monitoring a quantity of light is disposed in the vicinity of the first grating 1. Part of the laser beam LB is transmitted through the transmission section 17 for monitoring a quantity of light, enters the photo detector 16 for monitoring a quantity of light, and is converted into an electrical signal corresponding to the quantity of light. Electrical signals VA and VB from the measuring photo detectors 14A and 14B are sent out to calibrators 18A and 18B, respectively, these signals are calibrated by an electrical signal SL from the photo detector 16 for monitoring a quantity of light, and they are sent out to a signal/position converter 19 in which they are converted to position data.
According to the averaged diffraction moire position detector constructed as described above, even if the amount of light emission from a light source varies, or the transmittance of the first gratings 1 varies, no error will occur regarding a position to be detected.
In the above-mentioned conventional averaged diffraction moire position detector, since the position of the transmission section 17 for monitoring a quantity of light and that of the first gratings 1 are spaced apart, proper calibration is impossible in the case, for example, where transmittance is changed partially on the first gratings 1, or the transmittance of the second gratings 2 is changed. The conventional detector has another drawback in that the total size of the detector becomes larger since the transmission section 17 for monitoring a quantity of light is required.